The use of Niobium Tin (Nb.sub.3 Sn) wire in a superconducting magnet enables cooling of the magnet by means of a conventional Gifford-McMahon cycle refrigerator, which can efficiently produce refrigeration at a temperature of between about eight Kelvin (K.) to ten K. but is capable of cooling to lower temperature. It has been demonstrated that a superconducting magnet can be cooled by conductively transferring heat generated by the magnet through an aluminum shell to a Gifford-McMahon cycle refrigerator.
In a paper entitled "Applications of Superconductivity to Very Shallow Water Mine Sweeping" by E. Michael Golda, et al., published in the Naval Engineers Journal, May, 1992, a liquid helium cooled Niobium Titanium (NbTi) magnet operating at 4.2 K. is described and compared with a conductively cooled, Nb.sub.3 Sn magnet operating at 10 K. The cooling of the Nb.sub.3 Sn magnet is provided by a two stage, Gifford-McMahon, closed cycle refrigerator adapted for a mine sweeping application. While the weight of the conductively cooled, Nb.sub.3 Sn magnet and its insulation is less than the weight of the liquid helium cooled, NbTi magnet itself, the complete cooling system required for the conductively cooled, Nb.sub.3 Sn magnet still weighs more than the liquid helium cooled, NbTi magnet because of the added refrigerator. The refrigerator, however, frees the system from the logistical problem of periodically supplying liquid helium, often under difficult circumstances. Although the Golda, et al. paper is directed to usage in combat situations, the advantages of using a conductively cooled superconducting magnet also apply to other applications, such as in hospitals.
A paper by Geoffrey F. Green, et al. entitled "Conductively Cooling a Small Nb.sub.3 Sn Coil With a Cryocooler", presented at the 7th International Cryocooler Conference, Nov. 17-19, 1992, describes the construction and testing of a conductively cooled, Nb.sub.3 Sn magnet. The magnet wire is wrapped around an aluminum shell which is cooled by a two stage, Gifford-McMahon refrigerator. Heat is transferred into the aluminum shell from the support structure and the cold heat shield. The heat transfer through the aluminum results in a temperature difference between the warmest position on the magnet and the refrigerator of about 2 K. This temperature difference could be reduced by constructing the shell of the magnet from thicker and heavier aluminum material.
Another reference paper by Richard Stevenson entitled "50 kG Gas Cooled Superconducting", September, 1973, p. 524, describes a Nb.sub.3 Sn magnet operating at 13 K. which is cooled by a forced flow of helium gas through the windings. This is a more complex system than a conductively cooled magnet.
While conductive cooling has been demonstrated with a relatively small magnet where the wire is wrapped about an aluminum shell, as discussed above, it has not proven to be effective for a superconducting magnet constructed with a large coil wrapped about aluminum shell because of the difficulty in designing a suitable light weight shell.